G. Ottaviani

Intermetallic compound formation in thin‐film and in bulk samples of the Ni‐Si binary system

In this study, we attempt to clarify the difference between interdiffusion in thin‐film and in bulk samples, and we have chosen the Ni‐Si binary system for experimentation. A major difference is in the behavior of intermetallic compound formation; while the compounds form sequentially in thin films, they tend to appear simultaneously in bulk samples. The essence of sequential formation is the absence of the other stable phases during the growth of a specific one. We have used cross‐sectional lattice imaging of the interface between a NiSi film and a Si to confirm the absence of NiSi2 at the interface. Other differences between the sequential growth of a compound in the thin‐film Ni‐Si samples and the simultaneous growth of several compounds in the bulk Ni‐Si samples have been compared and discussed.

Thermal stability and growth kinetics of Co2Si and CoSi in thin‐film reactions

We have investigated the growth of Co2Si and CoSi around 400 °C in samples of Si/Co and Si/CoSi/Co. We selected the Co‐Si system because CoSi is known to have a larger heat of formation than Co2Si, hence the former should be favorable for formation from the viewpoint of free energy change or driving force. However, we found that Co2Si is the one which always grows first. This leads us to conclude that it is not the driving force but rather the kinetics which governs the selective growth of thin‐film intermetallic compounds. Thus, we propose here that the first phase formation is selected by the one with the lowest kinetic barrier.

Phase separation in alloy‐Si interaction

Phase separation has been found to be a general phenomenon among many alloy‐Si interactions at temperatures as high as 800 °C. We have studied these interactions with alloys consisting of a near‐noble metal of Ni, Pd, and Pt, and a refractory metal of Cr, V, and W. The interaction produces a silicide of the near‐noble metals next to the Si and an outer layer of silicide of the refractory metals. No evidence for ternary formation has been found.

NiSi formation at the silicide/Si interface on the NiPt/Si system

Alloy films of NiPt were e‐beam codeposited on n‐type Si and annealed up to 700 °C in a purified‐ He ambient furnace. Silicide formation was monitored using MeV4 He Rutherford backscattering and glancing‐angle x‐ray diffraction. At low temperatures (300–350 °C), Ni segregates at the Si/ silicide interface and the first phases detected are NiSi and PtSi. At intermediate temperatures (400– 500 °C), there is further accumulation of Ni at the Si/silicide interface, and at later stages an incursion of Pt to the interface. The barrier height increase reflects the presence of Pt. At 700 °C, the Ni and Pt redistribute to form a uniform ternary.

Temperature dependence of structural and electrical properties of Ta‐Si thin alloy films

In situ electrical resistivity measurement and structural analysis methods including MeV 4He+ ion backscattering, x‐ray diffraction, Auger electron spectroscopy combined with Ar+ sputtering, scanning and transmission electron microscopies have been used to investigate the properties and characteristics of the Ta‐Si thin alloy film system as a function of temperature. Stoichiometric TaSi2 films were deposited by double electron‐gun coevaporation on Si, oxidized Si, and sapphire substrates. A distinct drop in resistivity near 300 °C has been determined to be a transformation of the amorphous to crystalline phase. The kinetics of the transformation has been obtained by isothermal treatment over the temperature interval of 240–280 °C. The results are interpreted in terms of a classical solid‐state phase transformation model with a t4 (time) dependence and an apparent activation energy of 1.85 eV. Subsequent annealing causes further decrease of resistivity. The microstructures of films at various stages of ann...

Phase transformations of alloys on a reactive substrate: Interaction of binary alloys of transition and rare‐earth metals with silicon

Transition and rare‐earth metals have been found to interact with single‐crystal Si in a way that allows a division into three distinct classes: near noble, refractory, and rare earth. Recently, attention has turned to the reaction of their binary alloys with Si. In this paper we will try to demonstrate that by regarding the alloy‐Si reaction as a phase transformation of alloys under the influence of a reactive substrate, we can undertake a systematic approach for the study of this kind of phase transformations involving ternary elements. In essence we show that the kinetic path taken by the alloy‐Si interaction can be understood and anticipated from the reaction characteristics of the proper metal/Si bilayers and the reaction in the alloy itself. Results will be shown for Er‐Pt, and Gd‐Ti alloys on Si which confirm this systematic approach, which is also supported by previously published data.

In situ resistivity measurement of cobalt silicide formation

In situ resistivity measurements have been utilized to study the reaction and silicide formation between cobalt and amorphous silicon thin films from room temperature to 800 °C. In conjunction, structure and composition changes were analyzed by x‐ray diffraction and Rutherford backscattering spectrometry. Formation of Co2Si, CoSi, and CoSi2 were observed. Interfacial reaction to form Co2Si occurs at approximately 400 °C. In bilayers of excess silicon, CoSi forms at approximately 520 °C and, if free silicon is still present, CoSi2 forms at about 550 °C. In the case of excess cobalt, Co2Si forms first and is followed by a cobalt‐rich solid solution. Co3Si silicide was not observed.

Effect of substrate temperature on the formation of shallow silicide contacts on Si using Pd‐W and Pt‐W alloys

We have investigated the effect of substrate temperature on the reaction between Si and Pd‐W and Pt‐W alloys by I‐V measurement of Schottky barrier height. We found that by maintaining a substrate temperature of 100 °C during the deposition of W‐rich Pd20W80 alloys, Schottky contacts of Pd2Si on Si can be obtained without any subsequent annealing. Similarly, a substrate temperature of 300 °C enables the formation of PtSi during the depostion of W‐rich Pt20W80 alloys. For Pd‐rich Pd80W20 and Pt‐rich Pt80W20 alloys, the substrate‐temperature effect is not as significant as for the W‐rich alloys.

Interaction of Pd–Er alloys with silicon

In situ resistivity measurements together with MeV 4He+ backscattering, x‐ray diffraction, barrier height measurements, and Auger electron spectroscopy combined with Ar sputtering have been used to investigate the interaction of silicon with alloys of rare‐earth and near‐noble metals with annealing at temperatures up to 650 °C. Alloys of Pd–Er with three different compositions have been prepared by dual electron‐gun coevaporation on both n‐ and p‐type silicon and Pd/Er bilayers have been deposited on SiO2. The results show that as‐deposited these alloys are amorphous and the initial stages of the reaction with silicon upon annealing is controlled by the metal–metal interaction as well as the metal–silicon interaction. The Er‐rich alloy (Pd15Er85) segregates Er to the silicon interface and forms Pd2Er5. The segregated Er reacts with silicon producing ErSi2. For the Pd‐rich alloy (Pd65Er35) the excess Pd is segregated at the silicon surface forming Pd2Si. The near 50‐50 alloy forms PdEr and a slightly highe...